The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
A hand-held surgical instrument may have various parameter requirements including weight, power, size, current loss and heat generation requirements. Producing a hand-held surgical instrument with decreased weight, increased power output and/or torque, decreased size, reduced current loss and decreased heat generation can be difficult due to the relationships between these parameters. For example, as power output of a motor increases, heat generated by the motor generally increases. As another example, as size of a motor decreases, power output of the motor typically decreases.
A brushless direct current (DC) motor is capable of operating at higher efficiency levels and at higher speeds with reduced heat generation than a motor with brushes due to no mechanical contact between rotating and stationary components of the brushless DC motor. A brushless DC motor can include a rotor and a stator. The rotor has a shaft and a hub assembly with a centrally located magnetic structure. The stator has one or more coils. The coils are mounted on a centrally located support sleeve and proximally and distally located support rings. The rotor is held in a cavity of the stator and in a position relative to the support sleeve and support rings such that the rotor does not contact the support sleeve, the coils, and/or the support rings. Electrical current is supplied to the coils, which causes the rotor to rotate relative to the stator due to interactions between (i) magnetic fields generated by the coils and (ii) a magnetic field produced by the magnetic structure of the rotor. The rotor rotates axially in the support sleeve, the coils, and the support rings.
Brushless DC motors (hereinafter referred to as “motors”) convert electrical power into mechanical power or torque. During this conversion, losses can arise that limit mechanical power, torque and speed of the motors. These losses can generally be classified into three categories: (1) load sensitive losses dependent on generated torque; (2) speed sensitive losses dependent on motor speed; and (3) pulse-width modulation (PWM) losses dependent on quality of a current supply employed to drive the motors.
The load or torque sensitive losses are generally limited to winding losses, which are proportional to a product of (i) a square of an amount of current through windings of a motor and (ii) a resistance of the windings. Speed sensitive losses (e.g., core or iron losses due to Eddy currents and hysteresis, windage and friction) act as a velocity dependent torque opposite an output torque of a motor. PWM losses are attributable to Eddy currents in a magnetic structure caused by the current supply. Eddy currents are a phenomena caused by a variation of a magnetic field through an electrically conductive medium. In the case of brushless DC motors, the coils of the stator experience a change in a magnetic field. The rotation of the rotor and current variations in the coils induce a voltage in the coils, which results in the creation of Eddy currents. Increased Eddy currents can increase thermal energy output of a motor, especially when operating at high speeds.